In typical analog oscilloscopes of the prior art the voltage versus time behavior of the signal being observed is presented in real time on a cathode ray tube (CRT) display. An electron beam is moved horizontally across the display at a constant rate that is determined by a timebase setting. As the electron beam moves horizontally at this constant rate, the time-varying voltage level of the signal being observed controls the vertical position of the electron beam.
Even though the electron beam may move far too quickly to be perceived by the human, repetitive signals can still be perceived because of the persistence that is inherent in the light emitted by the phosphor coating of the CRT. Typically, for a repetitive signal to be visible to a human observer, the sweep across the CRT must be repeated at least 20 times per second. The actual sweep speed can be much faster, e.g., 10,000 or more updates per second. Depending on how much or how little a "trigger holdoff" control is applied, the signal being monitored may actually be visible on the face of the CRT up to 90% of the time or more.
The analog system just described has, however, one major limitation which is important to the present discussion, i.e., that rare, anomalous, non-repetitive events will usually go completely undetected, since by definition they are not repetitive enough to appear on the display the 20 times per second that is necessary for perception by the human eye. To compensate for this limitation, the display can be enhanced by the use of an electron multiplying faceplate, such as the microchannel plate system described in U.S. Pat. No. 4,752,714 to Sonneborn et al. for "Decelerating and Scan Expansion Lens System for Electron Discharge Tube Incorporating a Microchannel Plate" and U.S. Pat. No. 5,134,337 to Kongslie et al. for a "Projection Lens Assembly for Planar Electron Source", both of which are hereby incorporated by reference. An analog oscilloscope having a display enhanced by this microchannel plate technology can amplify a rare event to make it visible, so that such an event remains perceptible to the human eye for more than a second after only a single occurrence.
Unfortunately, microchannel plate technology is relatively expensive and, because of the high beam intensities that it generates, it is also prone to causing damage to the CRT phosphor unless the CRT is protected from over-exposure to the beam. When the intensity of such a system is turned up to view a rarely occurring signal anomaly, protective circuitry designed to avoid CRT burning will automatically reduce the intensity to avoid damage. This automatic dimming during operation creates a tension between the operator's desires and the display system's limitations, and this can be irritating and frustrating to the user.
In digital oscilloscopes the signal whose behavior is being monitored is sampled at regular intervals and each of these samples is quantized as a digital number that can be stored and otherwise processed before it is displayed. FIG. 1 is a simplified block diagram illustrating the data flow in a conventional digital oscilloscope, and FIG. 2 is a flow chart showing how the blocks of FIG. 1 operate.
Referring first to FIG. 1, incoming analog waveform data is quantized into numerical values by analog-to-digital converter 10 at regular intervals determined by an acquisition clock signal. These numerical values are stored in acquisition memory 20 at locations corresponding to successive time increments. Waveform processor 30 performs any desired manipulations of this data, rasterizes it, and stores the results in display memory 40. Display controller 50 accesses the contents of the display memory 40 and presents the resulting waveform on display 60.
As shown in FIG. 2, data is first acquired 70, and the old waveform is erased 72 from the display 40. Data representing the new waveform is then rasterized 74 and stored in the display memory 40. Finally, the display is refreshed 76 and the process is repeated again.
In a first type of digital oscilloscope, the quantized sample values are processed as desired and then converted back to analog voltages for display on a conventional CRT. In this type of system the maximum display update rate is about 100 times per second because considerable processing and display time is associated with each display cycle. If the sweep speed of such an oscilloscope corresponds to 10,000 records per second, but only 100 of these potential records are actually processed and displayed, that means that only one percent of the signal's behavior is available for viewing by the operator and 99% is lost from view. Such a characteristic seriously detracts from any possibility of finding an intermittent event of interest.
In a second type of display system for digital oscilloscopes, the display is stored in a digital bit map and presented on a raster scanning CRT display without ever being converted back into an analog signal. In this type of system the maximum display update rate is about 70 times per second because even a larger portion of the time is devoted to processing the digital data for each acquisition. Thus, for sweep speeds corresponding to 10,000 records per second, less than one percent of the signal's actual activity is available for viewing by the oscilloscope operator, so the chances of finding random anomalous signal behaviors is very small and when such behaviors are captured they are not visible to the human eye unless they happen to be stored and held for non-realtime viewing. The bit map type display can be made to behave more like a conventional analog CRT type display by causing the contents of the bit map to decay over time as newly acquired signal traces are added to it.
All of the display systems described above utilized cathode ray tube display systems, either to paint analog vectors or to provide a raster scan display. Yet, for many applications, newer display technologies are replacing the traditional CRT. Many of these newer display technologies provide a more rugged and compact alternative to the relatively fragile and bulky CRT.
Unfortunately, the newer display technologies that are currently available tend to be much slower than the CRTs that they replace. For example, liquid crystal displays (LCDs) now available are typically only able to be updated five times per second, i.e, only once every 200 ms. If a single newly acquired waveform is displayed with each such update, a very high percentage of the total sweeps that occur at even relatively slow sweep speeds will be lost during the interval between successive occurrences of these very slow display updates.
It would be highly desirable to have a way to provide oscilloscope users with much more information from all of the sweeps made by a fast quantizing data acquisition system, while at the same time providing the benefits associated with the more rugged and compact alternative display technologies now available. The invention described below provides such an oscilloscope display system and method.